Surveying is used for identifying subterranean elements, such as hydrocarbon reservoirs, freshwater aquifers, gas injection reservoirs, and so forth. Surveying can include seismic surveying or electromagnetic (EM) surveying. In seismic surveying, seismic sources are placed in various locations above an earth surface or sea floor, with the seismic sources activated to generate seismic waves directed into the subterranean structure.
The seismic waves generated by a seismic source travel into the subterranean structure, with a portion of the seismic waves reflected back to the surface for receipt by seismic receivers (e.g., geophones, hydrophones, etc.). These seismic receivers produce signals that represent detected seismic waves. Signals from seismic receivers are processed to yield information about the content and characteristic of the subterranean structure.
EM surveying involves deployment of one or more EM sources that produce EM waves that are propagated into the subterranean structure. EM signals are affected by elements in the subterranean structure, and the affected signals are detected by EM receivers, which are then processed to yield information about the content and characteristic of the subterranean structure.
Surveying of a subterranean structure can be performed in a marine environment, in which a marine vessel tows survey sources (e.g., EM sources or seismic sources) for generating survey signals to perform the survey. In many implementations, the survey receivers for measuring responses of the subterranean structure to the survey signals are dropped to the water bottom surface (e.g., sea floor). The survey receivers are dropped from the marine vessel into the body of water to reach the water bottom surface. In some implementations, each survey receiver has to reach the water bottom surface to within approximately 50 meters of the survey receiver's planned or target position. The actual location of each survey receiver has to be confirmed to be within 50 meters of its planned position before the next survey receiver can be deployed. The drop rate for survey receivers can be approximately half an hour per kilometer. In a deep marine surveying application, where the water depth can be several kilometers, it can take hours to drop each survey receiver to the water bottom surface.
After the survey receiver reaches the water bottom surface, the position of the survey receiver is typically confirmed by using an acoustic ranging system, such as an ultra-short baseline (USBL) system. The USBL system includes a USBL transceiver located at the marine vessel (such as at the bottom of the marine vessel) and a transponder on each survey receiver that is to be ranged. To obtain an unbiased estimate of the position of each survey receiver, measurements are typically made from several different angles to assure that the acoustically-determined distances are not biased (too long or too short). The process of waiting for each survey receiver to reach the water bottom surface and then estimating the position of the survey receiver with adequate geometry (at several different angles) can involve a substantial amount of time. If the survey arrangement includes a large number of survey receivers (hundreds or thousands), then the deployment of survey receivers can be very time consuming and costly.